The aim of this project is to further characterize the feline oncornavirus-associated cell membrane antigen (FOCMA) expressed on feline sarcoma virus (FeSV) transformed cells. Specifically, (1) the expression of FOCMA on nonproducer FeSV transformed cells of various species was defined by membrane immunofluorescence, (2) FOCMA was characterized biochemically to determine its size and whether there were determinants unique from the virus structural proteins, (3) hybridomas producing monoclonal antibody to FOCMA were developed in order to purify the protein and to employ the antibodies in an immunotherapy program for feline leukemia, and (4) to determine if FOCMA is a true tumor specific antigen and whether it is encoded by the virus and induced upon transformation independently of virus replication.